The present invention relates to a process for galvanic depositing of a dispersion layer on a surface of a work piece, in particular a contact layer on a plain bearing half liner, whereby an electrolyte with the dispersed phase finely distributed therein is moved opposite the work piece surface to be coated with formation of a flow component parallel to the surface.
The properties of dispersion layers can be adapted to various requirements by means of the dispersed phase, which comprises a wide range of particles at least substantially insoluble in the layer matrix, stored in the layer matrix. In the case of contact layers of plain bearings the mechanical and corrosion-inhibiting properties can be considerably improved by use of corresponding dispersion of hard material particles in the layer matrix. However, influence is brought to bear on the sliding behavior with the use of dispersions which are softer compared to the layer matrix.
In order to be able to apply dispersion layers comparatively simply to work piece surfaces, it is known to deposit these layers galvanically. For this purpose the particles of the dispersed phase are distributed finely in a corresponding electrolyte, where they are kept in suspension during the electrolysis process by stirring, air injection or pumping. The particles come into contact with the surface to be coated and are dispersed through adsorption, electrostatic attraction and through mechanical inclusion in the dispersion layer. The dispersion rate depends on the content of the dispersed phase in the electrolyte, yet higher dispersion rates require superproportional increases in concentration of the dispersed phase in the electrolyte, whereby the expense required for uniform distribution of the particles to halt their tendency to sedimentation rises as the proportion of the dispersed phase in the electrolyte increases. Corresponding limits arise in the attempt to increase the phase concentration in the layer matrix by augmenting the phase portion in the electrolyte.
Another possibility for increasing the dispersion rate of the dispersed phase in the layer matrix consists of increasing the flow rate of the electrolyte compared to the work piece surface to be coated. Such increase in the flow rate though only leads to an optimum because the dispersion rate drops again, presumably because of the rinsing effect. In this respect, it should be considered that the viscosity of the electrolyte sharply increases with the reduction in grain size in the dispersed phase, resulting in added difficulties. Especially fine-grained dispersed phases in the dispersion layer are preferred.
The object of the present invention is to create a process for galvanic dispersion of a dispersion layer on a work piece surface, in particular a contact layer on a plain bearing half liner of the type initially described, such that dispersion layers, which exhibit a comparatively high proportion of a dispersed phase, can be dispersed with comparatively little expenditure.
The invention solves this problem by the fact that the surface of the work piece to be coated is profiled prior to coating with an average minimum profile depth of 5 xcexcm and is then coated transversely to a relevant profile direction in a flow component of the electrolyte opposite the work piece surface.
Surprisingly, the portion of the dispersed phase dispersed in the layer matrix was able to be increased extraordinarily, without the necessity of augmenting the concentration of the dispersed phase in the electrolyte, by means of the profiling of the work piece surface to be coated with an average minimum profile depth of 5 xcexcm in conjunction with a flow component of the electrolyte opposite the work piece surface transverse to a relevant profile direction The processing expenditure going into the electrolyte with an increase in the phase portion is accordingly superfluous. Not only can higher dispersion densities of the dispersed phase be achieved in the layer matrix, but also normal phase concentrations in the layer matrix with a considerably reduced processing expenditure are achieved through significant reduction of the phase portion in the electrolyte. The effect of the considerably improved dispersing of the dispersed phase in the layer matrix on account of the profiling of the work piece surface to be coated can presumably be attributed to flow turbulence shifting in the vicinity of this profiling, which dispersions of the dispersed phase in the precipitating layer matrix decidedly support. For such a method of operation the flow component must run obliquely to this profile direction when the latter is salient. It is understood in this respect that profile direction means the course of more or less connected profile ridges with profile troughs in between, as is the case with profiling by creases or grooves. Plain bearings having a fluted bearing layer for taking up a galvanically dispersed contact layer are already known from AT 369 145 B, AT 382 215 B, and EP 0 155 257, but these profiled work piece surfaces bearing a contact layer do not serve an increase in the portion of a dispersion phase in the contact layer, which presupposes an electrolysis flow obliquely to the grooves or creases, but rather fine distribution of locally varying properties.
If the work piece surface to be coated is not profiled with a salient profile direction, as can be accomplished by etching or abrasive-blasting of the work piece surface with forming profile peaks for instance, no consideration needs to be made of the inflow direction of the flow component of the electrolyte parallel to the surface opposite the work piece surface, because there is no preferred direction with respect to profiling and corresponding turbulence in the vicinity of the profiling is to be reckoned with in all flow directions. It is not the orientation of the profiling that matters here, but the influence of the profiling on the electrolyte flow in the surface region.
Although in the case of an average minimum profile depth of 5 xcexcm a considerable increase in the dispersion rate of the dispersed phase can be established in the layer matrix, the desired effect can be substantially increased by significantly augmenting the average minimum profile depth. It is thus recommended that the surface of the work piece to be coated is profiled with an average minimum profile depth of 8 xcexcm. In choosing the average minimum profile depth it is natural to consider the surface configuration required for the respective work piece, which in turn requires reworking of the galvanically deposited dispersion layer
Since the dispersion rate of the dispersed phase in the layer matrix depends inter alia on the average profile depth, the content of the dispersed phase in the dispersion layer can be distributed differently over the work piece surface via the average depth in order to increase the stability of the contact surface in the peripheral region of the greatest stress of a plain bearing in comparison to the remaining areas, for example. For this purpose only the work piece surface to be coated needs to be profiled with various average profile depths. Another possibility of influencing the phase concentration in the layer matrix consists of blowing the electrolyte at different flow rates against the work piece surface to be coated in different areas, which also results in different properties of the deposited dispersion layer over the work piece surface. By controlling the flow rate of the electrolyte the phase content can be adjusted variously over the layer thickness. For this purpose only the flow rate of the electrolyte is altered compared to the work piece surface.
As already detailed, the fine grain size of the dispersed phase advantageous for the properties of the dispersion layer raises the viscosity of the electrolyte, such that the concentration of the dispersed phase has to be limited in the electrolyte for this reason. Despite this fact, dispersed phases with a grain size of less than 1 xcexcm, preferably less than 0.5 xcexcm, can be deposited finely distributed in adequate quantity in the dispersion layer, because the process can work with a comparatively small phase concentration in the electrolyte.